linux讀寫鎖實作

下面的代碼來自開源 sip項目pjsip，覺得實作的很簡潔，下次自己要實作的時候可以借鑒，于是就摘錄過來了。

該實作不支援同時多個線程寫。

//讀寫鎖的實作，值得借鑒
//不支援多個寫
struct pj_rwmutex_t
{
    pj_mutex_t *read_lock;//實際上是用來保護信号量的
    /* write_lock must use semaphore, because write_lock may be released
     * by thread other than the thread that acquire the write_lock in the
     * first place.
     */
     //write_lock必須為信号量，因為釋放write_lock的線程不一定就是最初
     //擷取write_lock的代碼位置
     /*
     互斥量的加鎖和解鎖必須由同一線程分别對應使用，信号量可以由一個線程釋放，另一個線程得到。
基于上面的特點，互斥鎖一般用于控制一段臨界代碼，當然信号量也可以做到。但是如果釋放和擷取不在一個函數中甚至不在一個線程中時就必須使用信号量了。
	*/
    pj_sem_t   *write_lock;
    pj_int32_t  reader_count;
};

/*
 * Create reader/writer mutex.
 *
 */
PJ_DEF(pj_status_t) pj_rwmutex_create(pj_pool_t *pool, const char *name,
				      pj_rwmutex_t **p_mutex)
{
    pj_status_t status;
    pj_rwmutex_t *rwmutex;

    PJ_ASSERT_RETURN(pool && p_mutex, PJ_EINVAL);

    *p_mutex = NULL;
    rwmutex = PJ_POOL_ALLOC_T(pool, pj_rwmutex_t);

    status = pj_mutex_create_simple(pool, name, &rwmutex ->read_lock);
    if (status != PJ_SUCCESS)
	return status;

    status = pj_sem_create(pool, name, 1, 1, &rwmutex->write_lock);
    if (status != PJ_SUCCESS) {
	pj_mutex_destroy(rwmutex->read_lock);
	return status;
    }

    rwmutex->reader_count = 0;
    *p_mutex = rwmutex;
    return PJ_SUCCESS;
}

/*
 * Lock the mutex for reading.
 *
 */
PJ_DEF(pj_status_t) pj_rwmutex_lock_read(pj_rwmutex_t *mutex)
{
    pj_status_t status;

    PJ_ASSERT_RETURN(mutex, PJ_EINVAL);

    status = pj_mutex_lock(mutex->read_lock);
    if (status != PJ_SUCCESS) {
	pj_assert(!"This pretty much is unexpected");
	return status;
    }

    mutex->reader_count++;

    pj_assert(mutex->reader_count < 0x7FFFFFF0L);

    if (mutex->reader_count == 1)
	pj_sem_wait(mutex->write_lock);

    status = pj_mutex_unlock(mutex->read_lock);
    return status;
}

/*
 * Lock the mutex for writing.
 *
 */
PJ_DEF(pj_status_t) pj_rwmutex_lock_write(pj_rwmutex_t *mutex)
{
    PJ_ASSERT_RETURN(mutex, PJ_EINVAL);
    return pj_sem_wait(mutex->write_lock);
}

/*
 * Release read lock.
 *
 */
PJ_DEF(pj_status_t) pj_rwmutex_unlock_read(pj_rwmutex_t *mutex)
{
    pj_status_t status;

    PJ_ASSERT_RETURN(mutex, PJ_EINVAL);

    status = pj_mutex_lock(mutex->read_lock);
    if (status != PJ_SUCCESS) {
	pj_assert(!"This pretty much is unexpected");
	return status;
    }

    pj_assert(mutex->reader_count >= 1);

    --mutex->reader_count;
    if (mutex->reader_count == 0)
	pj_sem_post(mutex->write_lock);

    status = pj_mutex_unlock(mutex->read_lock);
    return status;
}

/*
 * Release write lock.
 *
 */
PJ_DEF(pj_status_t) pj_rwmutex_unlock_write(pj_rwmutex_t *mutex)
{
    PJ_ASSERT_RETURN(mutex, PJ_EINVAL);
    pj_assert(mutex->reader_count <= 1);
    return pj_sem_post(mutex->write_lock);
}


/*
 * Destroy reader/writer mutex.
 *
 */
PJ_DEF(pj_status_t) pj_rwmutex_destroy(pj_rwmutex_t *mutex)
{
    PJ_ASSERT_RETURN(mutex, PJ_EINVAL);
    pj_mutex_destroy(mutex->read_lock);
    pj_sem_destroy(mutex->write_lock);
    return PJ_SUCCESS;
}
           

當然了linux應該針對線程已經有讀寫鎖的實作，不過我覺得徹底搞清楚實作會收獲更多，尤其是自己的實際解決問題，可能就需要用到别人好的實作思路。

如果自己要再實作的話，把裡面的信号量替換成線程的條件變量即可，實際上條件變量和信号量就是一個概念吧！